(i) Field of the Invention
This invention relates to rod-shaped polymer lattices comprising latex particles having a novel rod shape, rod-shaped polymer fine powders, and dispersions comprising the fine powder in non-aqueous media.
(ii) Description of the Prior Art
Polymer latices or emulsions have been widely utilized as binders for paints, adhesives, paper coatings, immunodiagnostics, sizes for clothes. These lattices are, in most cases, produced by emulsion polymerization and comprise particles which are spherical in shape. Recently, there has been proposed a method of preparing a polymer latex other than the emulsion polymerization in which a polymer latex called soap-free emulsion is prepared without use of any surface active agents. In this case, the particles are nothing but spheres. Thus, the shape of the particles currently utilized as polymer lattices is spherical in all cases. On the other hand, attempts have been recently made to change the shape of latex particles. In fact, there have been proposed several odd-shaped latices. These particles can be broadly classified according to the shape thereof as follows.
(1) Core-shell type particles in which polymer particles have different components of the core and shell (Japanese Laid-open Patent Application No. 55-55414).
(2) Confetti particles in which polymer particles have irregularities on the surfaces thereof (Collection of Polymer Theses, Vol. 36 (1979)).
(3) Localized particles in which different types of polymers suffer phase separation in a latex (Kagaku Giken Report Vol. 35, 1980).
In all the cases, however, spherical latex particles are provided as nuclei and are covered with arbitrary polymers by seed polymerization. Accordingly, latex particles are considered to be spherical particles of latices which are somewhat varied or modified in shape. According to the conventional polymerization methods, there could not be obtained lattices comprising particles which are not spherical in shape but are so shaped as to enhance the function thereof.
There is a high demand for techniques of imparting higher performance to lattices by changing the shape of lattices particles. For instance, when polymer lattices are used as paint binders, good thixotropic properties are required. With known spherical lattices, however, such a requirement cannot be satisfied, so that it is usual to add other types of polymer compounds capable of imparting thixotropic properties to the lattices. However, if it is possible to permit paints to develop thixotropic properties by changing the shape of latex particles, addition of the thixotropy-imparting polymers becomes unnecessary. With diagnostic lattices, latex particles having a large surface area are required in order to increase an adsorption of antigen or antibody and enhance the sensitivity to immune reaction. As is well known, sphere is in such a shape that has the smallest specific surface area. In this sense, spherical latex particles are not suitable for use in lattices for diagnosis.
Rod-shaped inorganic fine powders such as of kaolin, goethite, .gamma.-iron oxide, wollastonite, light calcium carbonate, and the like have been widely utilized as pigments or thickeners for make up cosmetics, paints, paper coatings and the like. However, these materials are all natural inorganic materials, so that their properties are substantially similar to one another and there has never been known any material which is substantially different in nature from other materials. With regard to surface properties, for example, these known materials are hydrophilic in nature. Accordingly, it is the usual practice that if hydrophobicity is required as with loading pigments for makeup cosmetics, the pigment is inevitably treated to impart the hydrophobicity thereto. Moreover, the inorgannic materials are great in specific gravity, so that when they are used as dispersions in non-aqueous media (e.g. oil paints), it is necessary to prevent the materials from setting. In addition, these natural products have the problem that a constant quality cannot be ensured. For instance, with wollastonite, their major axis distributes from 1 .mu.m to about 500 .mu.m. Moreover, because of the presence of impurities, these natural rod-shaped inorganic powders have to be purified through a number of steps, with various problems accompanied at the time of effective utilization of these natural fine powders.